Bosch supplies assist units for some amazing road bikes that have
stunning new e-shift technology, intuitive pedal assist and good battery
life. For serious all-terrain use, a mid-drive is all but mandatory.

MID-DRIVE

With a mid-drive, the basic design of
the motor is the same: iron wrapped with
copper windings moving near magnets.
In the case of mid-drives, though, the
outer motor shell is secure and the center
shaft spins. Usually, the mid-drive uses
an even smaller, higher rpm motor than
a geared hub motor, and it uses even
less copper and magnets to get the
job done. Most employ a double-gear
reduction to get the rpm of the smaller
motor in the ballpark and to multiply
the torque. In many cases a mid-drive
assist motor requires a motor-specific
frame with a motor mount rather than a
bottom bracket for the crank. There are
mid-drives that fit in a standard bottom
bracket. They completely replace the
bicycle crank.

ADVANTAGES

Motor uses the gears: On a hub
motor you can only change the motor
gearing by changing the size of the
wheel. Mid-drives allow the motor to
have more gearing choices.

Central mass: The assist motor
and often the battery are located in the
middle area of the bike so the handling
isn’t compromised with a lot of weight at
either end.

Standard wheels: A mid-drive allows
the bike to use standard bicycle wheels
so you can get parts or service anywhere,
and changing a flat during a ride is a
snap.

Cadence rather than speed-based:

If you want a hub motor to climb strongly,
you must stay above a certain speed.

For the mid-drive you must simply be in
the right gear. If you have low-enough
gearing, it will climb as strongly at 3–4
miles per hour as it does at 13–14 mph.

Economy: By having the right gear for
the job and using a smaller motor inside,
a mid-drive is often better on battery use
than a hub motor.

Heat management: With the small
motor and gearing options, we have
never overheated a mid-drive with
extended climbing.

DISADVANTAGES

Motor uses the gears: You are
adding power to the chain and cassette,
so they can and will wear faster than with
a normal bike or a hub motor. Shifting
may also be less smooth and noisier than
with a hub motor.

Gear selection is more important:

You must ride a mid-drive more like a
bicycle. If you get lazy and forget to shift
to a lower gear before stopping, taking
off can be a real chore.